Picture transmission, television, and the like



V. J. TERRY May 5, 1942.

PICTURE TRANSMISSION, TELEVISION, AND THE LIKE 2 Sheets-Sheet 1 FiledSept. 8, 1958 ESQ EmN

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IN VENTOR k'J. TERRY 2 Sheets-Sheet 2 V. J. TERRY Filed Sept. 8, 1938PICTURE TRANSMISSION, TELEVISION, AND THELIKE May 5, 1942.

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was mm m ww SmEqu Patented May 5, 1942 PICTURE TRANSMISSION, TELEVISION,

AND THE LIKE Victor John-Terry, Aldwycli, London, England,

assignor to Western Electric Company Incorporated, New York, N. Y.

Application September 8, 1938, Serial No; 228,993 In Great BritainSeptember 10, 1937.

4 Claims.

This invention relates to image current transmission and moreparticularlyto the production and utilization of aform of such currentparticularly useful for transmission over lines. and through amplifiers,modulators, networks, filters, etc.

Anobject of the invention is to provide a novel method and means forgenerating an image current in which the direct and very low frequencycomponents resulting from the scanning operation are effectivelyeliminated as such and in which the values of these components arerepresented by other components, and a further object is to providesimple, means for generating from said current another current in whichsaid direct and low frequency components as such are present.

A further object is to provide a novel method of and means for producingsynchronizing signals of very precise form as a part.of, a televisionpicture transmission current.

Other objects and advantages of the invention will be apparent from the.following description:

Theprinciple of the invention is to commutate .the current whosefrequency characteristic it is desired to modify, by switches or amodulating circuit at regularly recurrent intervals, so choosing thefrequency-and phase of the instants of commutation, that they occur attimes when the value of the current'is unimportant or is of a knownvalue duced.

In the case of a picture current, suitable moments for commutationusually occur between the end of every line and the beginning of thenext, for here the current value is usually unimportant: in the case ofcertain well known systems of television suitable moments occur in theintervals. between lines and between frames. In each of these intervalsthere is norwhich may subsequently. be repromally a period (or periods)when the current has :without appreciable alteration in the total range.

infinitely to the frequency content of the current, unless it takesplace at an instant when the current is zero in which case it may eitherreduce or increase the range of the Fourier components.

When the commutation process takes a finite time, as when it isaccomplished by modulation with a wave of squarish but not perfectlysquare wave form, it is desirable to arrange that the squarish wave issufficiently rounded, or the commutation sufficiently slow, to ensurethat the current changes it produces are not more rapid than otherchanges of amplitude inherent in the wave, and the tendency of thecommutation process is then to move the frequency range bodily upward byabout half of the line frequency,

An indication of the nature of these frequency changes is obtained byconsidering the energy frequency distribution of a typical televisioncurrent. ,The composition of a typical television signal is shown in anarticle by Pierre Mertz and Frank Gray in Bell System Technical Journal1934 at page 478.

It will be found to contain a strong zero frequency component (D. C.).There will also be strong components of the order of a fraction of ahertz but practically no energy above one hertz until the framefrequency (usually 25 hertz) is approached. Thereafter, there are rangesof practically zero energy, separated by regions of small (butappreciable energy corresponding to the lower harmonics of the framefrequency. These grow weaker and are almost negligible after about thetwentieth leaving a gap of zero energy until the line" frequency(usually about 10 k. c.) is nearly reached. The energy distribution,both above and below this frequency corresponds approximately to that inthe first few hundred cycles above zero.

Around each harmonic of the line frequency is centred a similar energyfrequency group.

Between each of the first few groups there is a wide range of almostzero energy, but the roups corresponding to the higher harmonics, thoughweaker in energy, are more diffuse in frequency distribution. Towardsthe upper. end of the frequency range, the groups are no longer welldefined and the energy distribution is almost uniform.

The effect of the modulation process is to replace every formercomponent present by a series of components. Two in the seriescorresponding to a single component differ from the original by thefundamental of the modulation frequency and two more differ by each oddharmonic thereof.

The fundamental of the modulation frequency for one commutation betweeneach line is half of the line frequency so that the total effect of themodulation on the complex television current is to replace the groupoccurring at and immediately above zero frequency (up to the framefrequency and stronger harmonics thereof), and the groups centred aroundthe line frequency, and harmonics thereof, by other groups centredaround half the line frequency and odd harmonies thereof. Each of thesenew groups is a combination of one of original groups raised by half theline frequency and another reduced by half the line frequency plussmaller additions from other groups raised or lowered by odd mul- Itiples of half the line frequency.

Components of the modified wave which lie close to zero frequency canonly be derived from components in the original wave which lay mid- Waybetween zero and the fundamental line" frequency or mid-way betweenadjacent harmonies of the line frequency. In all these regions thecomponents are normally so weak that they may be eliminated withoutappreciable error.

Therefore for the transmission of the modified wave, it is not necessaryto transmit the lowest frequencies. I

When, owing to the size of the scanning spot or to the use of selectivecircuits, the frequency of the highest components in the unmodifiedsignal is limited, the usual effect of the commutation is to extend therange upward by the modulating frequency and to add unimportantcomponents extending-several times the line frequency above the originallimit. This is usually a negligible addition.

If,.on the other hand, the components of the original Wave, are notlimited in this way but extend above the efficient range of thetransmission path, some of the components in the original Wave whichmight have been transmitted are translated upward beyond the range ofefficient transmission. Others on the other hand, whichwouldhave'fa'llen beyond the transmission range are translated downwardsinto it and are thus rendered usable. The net result is therefore thatfor a given upper limit of frequency, the commutating process involvesno loss of detail, if the frequency limitation is applied after themodification. The restoration of the original wave is very easilyaccomplished by full wave rectification.

If the commutations occurred at a moment of zero current, therestoration may be perfect, but if at instants when the current isfinite, then the instants are marked by short periods during which thecurrent falls to zero and rises again.

These short period falls need not prove objec tionable because theyoccur between the lines of the vision or picture.

In a system for transmission it is convenient to effect the commutationat the earliest possible moment and to retain the current in themodified form as long as possible.

Thus in a system employing for transmission a linear-scanning devicefollowed by an amplifier of the electron multiplier type (whichamplifies all freqi encies down to zero) a line amplifier (transmittingonly alternating current), a transmission line, a further line amplifier(like the first), and a radio transmitter, the commutating device shouldpreferably follow immediately after the electron multiplying amplifier.Rectification should be applied (if at all) immediately before the firstmodulator in the radio transmitter.

It is, however, preferable to omit the rectifier and to employ amodulator in the radio transmitter, which gives double side bandtransmission with the carrier suppressed.

The envelope of the carrier wave is then exactly the same as if themodified wave had been restored by rectification, but at each instant ofcommutation the phase of the carrier is reversed. This reversal does notaffect the ordinary television receiver, but if signals having the phasereversed are detected with a synchronised local oscillator, thecommutated Wave is reproduced, which is convenient for amplification andfor the control of the synchronising circuits.

To take full advantage of the convenience of the modified wave as acontrol for the synchronising circuits, it is desirable to eliminateentirely from the uneommutated wave the zero amplitude period, leaving aconstant amplitude period between the actual vision periods.

If the commutation is carried out during the constant amplitude periodat a rapid but not necessarily infinite speed, a very precise indicationis obtained for synchronising, and the end of frame (which is normallymarked by a succession of line periods during whichthe current value isvaried in accordance with a predetermined code between zero and somevalue equal to or less than the lowest picture value), can b indicatedby several line" periods having a lower current value than anyassociated with the actual vision period, or by the insertion ofadditional commutations or both.

If additional commutations are inserted, the total during any lineperiod should preferably be odd, in order to eliminate the directcurrent component as completely as possible,

When using the commutation principle it is desirable that the number ofactual lines per frame, and the number of blank lines betweenframesshould both be even, and it is preferable that the actual lines shouldboth start and finish with a half line.

All these conditions are easily arranged in a plain scanning system, butin an interlaced system something must be sacrificed. The number ofactual lines in each half frame may still be even, but the number ofblank lines must then contain a fraction, and if half an actual line isused to start and finish one half frame, the other half frame cannotreadily be made to do the same.

Departure from these conditions introduces a very small steady D. C. orframe frequency component which may normally be neglected. A steady D.C. component may, balanced out very easily.

When two side bands of the commutated wave have been transmitted, forexample by radio, and, for the sake of precise synchronisation inspecial receivers, the normal synehornising intervals of zero currenthave been omitted, it is advantageous to make the rate of commutationmoderate, for then the envelope of the transmitted side bands is for anappreciable time approximately of zero amplitude and the interval ofapproximately zero amplitude will be accepted by ordinary receivers as asubstitute for the normal zero amplitude synchronising pulses.

The invention will be further described as embodied in a particulartelevision transmitting sysof course, be

tern, and with reference to the accompanying drawings. In the drawings:

Fig. 1A is a diagram showing the wave-form of a television signalproduced in known manner for example by the apparatus described in anarticle hereinafter mentioned.

Fig. 1B is a diagram showing one way in which the wave form of Fig. 1Amay be commutated in accordance with the invention.

Fig. 2 is a block circuit diagram incorporating features of theinvention and capable of givingthe wave form'of Fig. 1B, and

Fig. 3 is a more detailed circuit diagram of a modulating circuitsuitable for use in Fig. 2 in the case where the input line to themodulating circuit is balanced. I

Referring first to Fig. 2, this figure shows how the invention may beapplied to the television transmitting system described in a paper byMessrs. Blumlein, Browne, Davis and Green, read before the Institute ofElectrical Engineers on April 21, 1938. The figure is based on Fig. 3 ofthat article. Here the scanning device or camera I produces the visionwave subject to several varieties of distortion called shadingdistortions which are corrected by means of a shading control in thefirst amplifier 2. Subsequent mixers 3, 4 and 5 suppress parts of thewave and add further impulses for synchronisation. in this case thecommutation process of the invention should not be applied until thecorrection and mixing are complete, unless the phase reversals are to beused for both line and frame synchronising controlsin which case thesyn-.

chronising mixer 5 is omitted.

The picture channel of the transmitter includes" additional amplifiersat 6 and a distribution am- The various generators 9 to I3 arecontrolled as described in the aforesaid article by a line masterfrequency source It and a frame master frequency source [5 which arecontrolled in turn over frequency dividers ll, l8 by a master oscillatorI" of twice line frequency. The divider. 11 effects a division by twoand the divider 18 provides various divisions as necessary for the framesource I5.

In order to adapt the system for the commutation of the picture wave inaccordance with the invention a commutating modulator 2D is inserted infront of the distribution amplifier 8 and is controlled by a modulatingwavegenerator fl provided for that purpose. The frequency ap plied tothe phase reversing commutator or commutating modulator 20 is assumed tobe half the line frequency (giving one commutation per line). Anadditional frequency divider 22 is therefore provided which operatesfrom the line I The usual wave form applied to the distributionamplifier 8 is shown in Fig. 1A. It includes line synchronisingintervals of substantially zero amplitude and framesynchronising signalsextending for the duration of several lines and having two pulses perline interval as shown.

The effect of commutation at line frequency and during the linesynchronising intervals is to signals of alternate lines are negative.It has been assumed in Fig. 13 that the commutation is instantaneous butit will be clear that the commutation may take a-finite time within thesynchronising interval.

A simple circuit for the commutating process is shown in Fig. 3. Itconsists of the well known metal rectifier ring modulator M suppliedover transformer Tl by a carrier wave at half the line frequency. Thecommutated or modulated wave is taken from transformer T2 and theunmodulated wave is applied over line L. For rapid and accuratecommutation the carrier wave is given an approximately square outline bypassage through an overloaded push-pull amplifier P to suppress thepeaks, the positive and negative half cycles of the output being equalin duration and the amplitudes of the positive and negative half cyclesbeing constant during the whole of the periods during which theamplitude of the picture or television wave corresponds to parts of thescene (about 85% of each half cycle). The amplifier P constitutes themodulating wave generator 2| of Fig. 2 and receives its input from thedivider 22. The ring modulator M is illustrated as being thesimplesttype fulfilling the essential requirements that (a) the inputtransmission path must be capable of transmitting all frequencies downto zero, (b) there must be complete suppression in both input and outputcircuits of the commutating wave and (c)- there must be completesuppression in the output circuit of the uncommutated input wave.

The ring modulator is not however, to be regarded as .necessarily bestfor the purpose and other suitable forms of modulator will be readilyappreciated.

vWhat is claimed is:

.1. Television transmitter comprising means for scanning elementallinesof a field of view in succession to produce an image current, and meansincluding commutating means for reversingdn sign with respect to thepreceding group every group of signal waves corresponding to thelighttone values of successively scanned lines or pairs or groups oflines, thereby eliminating or weakening the direct current component andthe lower frequency components of the signal wave arid producingcomponents indicative of said eliminated or weakened components.

2. Means for scanning elemental .lines of an object or object field insuccession with relatively short intervals between said line scannings,

" means including said scanning means for setting up a single unitaryimage current having variations corresponding to the light-tone valuesof said lines in the order in which they are scanned, said current beingalternately oppositely directed with reversal of direction taking placeonly at times corresponding to said short intervals and Y such that eachperiod of unidirectional flow give the wave form shown in Fig. 11!,where the represents the light-tone value of an integral number of saidelemental lines, whereby said current over a period corresponding to thetime of scanning a multiplicity of said lines in succession has nodirect or very low frequency component of more than negligibleamplitude, and means for impressing said current upon a transducingelement which cannot correctly transmit a continuously direct imagecurrent representative of successively scanned lines because of itsinability properly to transmit direct and very low frequency components.

3. Means for'scanning elemental lines of an object or object field insuccession and for setting up a direct image current representative ofthe light-tone value oi' the successively scanned lineshavinglarge-amplitude components at line scanning frequency and integralmultiples thereof, the frequency spectrum oi said current having regionsmidway between adjacent ones oi! said components where there are nocomponents of appreciable amplitude, said current'also havinglarge-amplitude direct or very low frequency I 7 componentsrepresentative of the average lighttone value of a multiplicity oisuccessively scanned lines, a transducing element incapable of properlyhandling said current because of the presence therein of the direct orlow frequency components, and means for receiving said current andtransmitting it to said transducing elea ment comprising means fortransforming said current into a second current similar in amplitudevariations to said first current but having alternate portions of equalduration oppositely directed each consisting of direct current only andrepresenting the light-tone values of an integral number of successivelyscanned lines, said second current having no components of appreciablevalue corresponding to said direct or very low frequency components ofsaid first current.

4. The combination with means for scannin elemental lines oia field ofview in succession toobtain an image current made up of succeedingportions respectively representative of the light-tone values of thesuccessively scanned elemental lines, of means for causing alternateones of said portions to be opposite in polarity from A the intermediateones of said portions, and means for producing in said current oifreversed polarity as a part of each of said portions and at the endthereof a relatively short portion of constant amplitude, throughoutsaid current, and of the same polarity as the portion which itterminates.

VICTOR JOHN TERRY.

